Seven years ago this month the Kepler spacecraft launched into space – the first NASA mission dedicated to searching for planets around distant stars. The goal was to conduct a census of these exoplanets, to learn whether planets are common or rare. And in particular, to understand whether planets like Earth are common or rare.
With the discovery and confirmation of over 1,000 exoplanets (and thousands more exoplanet candidates that have not yet been confirmed), Kepler has taught us that planets are indeed common, and scientists have been able to make new inferences about how planetary systems form and evolve. But the planets found by Kepler are almost exclusively around distant, faint stars, and the observations needed to further study and characterize these planets are challenging. Enter TESS.
The Transiting Exoplanet Survey Satellite (TESS) is a NASA Explorer mission designed to search for new exoplanets around bright, nearby stars. The method that TESS will use is identical to that used by Kepler – it looks for planets that transit in front of their host star. Imagine that you’re looking at a star, and that star has planets around it.
If the orbit of the planet is aligned correctly, then once per “year” of the planet (i.e. once per orbit), the planet will pass in front of the star. As the planet moves in front of the star, it blocks a small fraction of the light, so the star appears to get slightly fainter. As the planet moves out of transit, the star returns to normal brightness. We can see an example of this in our own solar system on May 9, 2016, as Mercury passes in front of the Sun.
We can learn a lot from observing the transits of a planet. First, we can learn the size of a planet – the bigger the planet, the more light it will block, and the larger the “dip” in the brightness of the host star. Second, we can learn how long the planet’s year is – since it only passes in front of the star once per orbit, the time between transits is the planet’s year.
The duration of the year, in combination with the properties of the host star, also allows us to determine if a planet might be habitable. With high precision measurements, we can also infer much more about the orbit of the planet (e.g., the eccentricity of the orbit). And, in fact, in some cases, we can look at small changes in the apparent year of the planet to discover additional planets in the system that do not transit (Transit Timing Variations).
To observe these transits, TESS will use four identical, extremely precise cameras mounted behind four identical 8-inch telescopes. Each one of these cameras will be sensitive to changes in the brightness of a star as small as about 40 parts per million, allowing TESS to detect planets even smaller than our planet.
Earth, transiting the sun, would produce a dip of about 100 parts per million. Each of the four cameras has a field-of-view of 24°×24°, and the fields of the four cameras are adjacent so that TESS will instantaneously observe a 24°×96° swath of the sky (referred to as an observation sector). Within this field, TESS will collect “postage stamp” images of about 8,000 stars every two minutes – the postage stamps are small sub-images, nominally about 10×10 pixels.
TESS will stare continuously at each of these observation sectors for 27 days before moving to the next sector; over the course of one year, this will give TESS coverage of almost one entire hemisphere, with postage stamp data on approximately 100,000 stars. In the second year of the TESS mission, 13 additional sectors will cover the other hemisphere of the sky, resulting in observations of about 200,000 stars.
The method used for these postage stamp-sized observations is very similar to that used for Kepler, but the survey itself is different. While TESS is conducting an all-sky survey (about 40,000 square degrees), Kepler looked at only a relatively small patch of the sky (115 square degrees). But with a telescope seven times larger than those on TESS, Kepler was able to look much further away – TESS surveys stars within only about 200 light years, compared to 3,000 light years for Kepler.
This underscores the difference in the underlying philosophy of the two missions. The goal of Kepler was to understand the statistics of exoplanets, to conduct a census to understand the population as a whole.
TESS, on the other hand, is about finding planets around bright, nearby stars –planets that will be well-suited to follow-up observations from both the ground and from space. On average, the stars observed by TESS will be between 30 and 100 times brighter than those observed by Kepler. These brighter targets will allow for follow-up observations that will be critical for understanding the nature of the newly discovered planets – more on that in a moment.
In raw numbers, what do we expect from TESS?
Former MIT graduate student Peter Sullivan conducted detailed simulations of the mission to make a prediction on what it might discover, and these results are incredible. With TESS, we expect to find over 1,600 new exoplanets within the postage stamp data, with about 70 of those being about the size of the Earth (within 25% of the Earth’s diameter), and almost 500 “super-Earth” planets (less than twice the diameter of Earth).
Perhaps most exciting is the likelihood that TESS will discover a handful of Earth-sized planets in the habitable zones of their host stars.
In addition, while TESS obtains the postage stamp data every two minutes, it also obtains a full-frame image – a picture of the entire observing sector – every thirty minutes.
In those data, we expect to find over 20,000 additional planets. The majority of those will be large (Jupiter-size) planets, but there will also be about 1,400 additional super-Earths discovered. The sheer number of planets that will be found is amazing, but more important than the number is the fact that all of these planets will be orbiting bright, nearby stars. This is a fantastic leap relative to where we were just 25 years ago, when not a single exoplanet was known.
One of the challenges of transit measurements is that they can produce false positives. Stellar activity can cause quasi-periodic dips in the brightness of a star. An eclipsing binary star in the background could mimic the dip from a transiting planet. With careful analysis, most of these effects can be accounted for, but it remains important to follow a transit observation with a confirmation — making a secondary measurement to ensure that what was observed is, in fact, a planet.
The most straightforward way to confirm a transiting exoplanet is with a radial velocity (RV) measurement. The RV method takes advantage of the reflex motion of the star; as a planet orbits a star, the star itself doesn’t remain stationary. In fact, both the planet and the star orbit the center of mass of the system. So, if one looks at spectral lines from the host star, it is possible to measure the Doppler shift of those lines as the star does it’s little pirouette around the center of mass.
From this data, astronomers can measure the mass and the year (orbital period) of the exoplanet. This confirms the orbital period observed from the transit data, and the combination of radius (observed from the transit) and the mass (observed from the RV) gives us the bulk density of the planet. With that, we can make inferences about the composition of the planet – is it a rock, like Earth? A water-world or a ball of ice? A gas giant?
Making the RV measurement, while straightforward, is not an easy one – less than 10% of the exoplanet candidates found by Kepler have been confirmed with RV measurements, largely because the host stars themselves are faint. For TESS, however, because the host stars are nearby and bright, it will be possible to make follow-up observations on nearly all of the stars that host small planets – the only major limitation will be due to the noise from the stars themselves (i.e. flares, starspots).
Further, because these host stars are bright, they will also be excellent targets for transit spectroscopy. Imagine, for a moment, that there is a transiting planet with a very large atmosphere, and that this atmosphere is transparent in red and blue, but completely opaque in the green. Then, if you observe the planet in red light (or blue light), only the “rock” part of the planet will block light from the star. In green light, however, the rock and the atmosphere will both block light – in the green, the planet appears to be larger than at other wavelengths.
This is the core idea behind transit spectroscopy. By measuring how the apparent size of a transiting planet varies with wavelength, we can infer the composition (and potentially the structure) of the planetary atmosphere. This technique has been used successfully on a very small number of exoplanets to date, but with the large number of planets that TESS will find, and the fact that they will all be around bright, nearby stars, it will be possible to use the James Webb Space Telescope and the next generation of large ground-based telescopes to make these observations.
For the first time, astronomers will actually be able to study not only individual exoplanets, but will be able to study enough of them to make comparisons and draw conclusions about how planets form and evolve.
For me, TESS is endlessly exciting. The sheer quantity of new exoplanets is stunning. The ability to use follow-up observations to characterize these planets will create new paths for scientific investigation. And the discoveries made will help define the science that will be pursued by future missions such as WFIRST, and perhaps more ambitious missions in the future. But, perhaps most exciting, TESS is in part about making “Exoplanets for Everyone.”
In a few years, it will be possible for everyone to go outside to a dark location, point at a star that you can see with the naked eye, and say “there is a planet around that star.” And the night sky may never feel quite the same again.
Video link: TESS Trailer — https://youtube/ZsPStvGgNuk